Carbonate esters are useful compounds as a raw material for synthesis of aromatic polycarbonates, and drugs and agricultural chemicals, etc. Oxalic esters are also useful compounds as a raw material for synthesis of glycols, dye intermediates, and drugs, etc. Methods for continuous mass production of these compounds have been previously proposed.
As a process to continuously produce a carbonate ester, a process is known in which synthesis through gas phase reaction is performed by using carbon monoxide and a nitrite ester in the presence of a solid catalyst of a platinum group metal (e.g., see Patent Literature 1). In such a process, a carbonate ester can be obtained through the following reaction formula (i).CO+2RONO→ROC(═O)OR+2NO  (i)
NO obtained through the reaction represented by the formula (i) reacts with an alcohol in accordance with the following formula (ii) to form a nitrite ester in a reaction column (regeneration column). By reusing the nitrite ester obtained in the reaction and newly feeding carbon monoxide, a carbonate ester can be continuously produced. An oxalic ester can be also produced continuously in a similar process (e.g., see Patent Literature 2). Thus, carbonate esters and oxalic esters are produced by circulating nitrogen components such as a nitrite ester.2NO+½O2+2ROH→2RONO+H2O  (ii)
In the process as described above, a reaction represented by the formula (iii) also proceeds in a reaction column. HNO3 formed in the reaction is extracted from the bottom of a reaction column. Patent Literature 3 proposes to introduce HNO3 thus formed into a nitric acid-concentrating column for effective use of nitric acid to reduce loss of nitrogen components.NO+¾O2+½H2O→HNO3  (iii)